The invention relates to a device for the compensation of hearing impairments. Devices of this type are disclosed, for example, in Scand. Audiol. 8:121-126, 1979, as "Programmable Hearing Aid With Multi-Channel Compression" by S. Mangold and A. Leijon (cf., in particular, page 121, right column, last paragraph up to and including page 122, right column, paragraph 4).
In the known device, the electrical input signal which is generated, for instance, in a microphone or in an induction pick-up coil, is supplied to a plurality of filters which respectively allow adjacent frequency bands of the input frequency range to pass.
The individual frequency bands derived from the input signal are then transmitted via parallel signal channels having non-linear signal processing circuits such as compression circuits for compressing the dynamic range, and amplitude attenuation circuits. Finally the signal components from the respective parallel signal channels are combined into a resultant signal and supplied to the ear of a hearing impaired person via an output transducer. The parameters for the filters, the dynamic range compression circuits and the amplitude attenuation circuits may be supplied from a digital memory which has been programmed based on data concerning an individual's hearing impairment. For example, the data may be obtained from an audiometer and supplied to a data input of the hearing aid.
Although the analog signal processing realized in the known device represents a fundamentally simple method and corresponds to the technology hitherto employed in hearing aid technology, the following disadvantages derive in the realization of the apparatus:
(1) If the hearing aid is also to be able to compensate for serious hearing impairments (for example great loss of high tones), then filter circuits are necessary which require a great deal of space and power, so that incorporation in behind-the-ear devices is made more difficult.
(2) Precision and temperature stability problems in the resistors and capacitors results, particularly when the filters are to be realized in integrated circuit technology.
(3) The setting of the filter characteristic with the variation range and precision necessary for a universally employable hearing aid requires very involved circuits (for example digital-to-analog converters and analog multipliers).
The disadvantages cited under (2) and (3) are avoided when the signal processing is carried out completely digitally, i.e. time-discrete and amplitude-quantized. Such a hearing aid functioning with integrated logic circuits is known from U.S. Pat. No. 4,187,413. Because of the outlay for the analog-to-digital converter at the input and the digital-to-analog converter at the output, the difficulties cited above under (1) remain. In particular, the high power consumption of such circuits can only be accommodated with difficulty from the batteries employable in the installation space already limited by the circuit in behind-the-ear devices.